This disclosure generally relates to coatings and components for high temperature applications, such as gas turbine assemblies.
The design of modern gas turbines is driven by the demand for higher turbine efficiency. It is widely recognized that turbine efficiency can be increased by operating the turbine at higher temperatures. In order to assure a satisfactory life span at these higher temperatures, thermal barrier coatings (hereinafter referred to as “TBCs”) are applied to airfoils and combustion components of the turbine—such as transition pieces and combustion liners—using various techniques.
A key concern for turbines used in both power generation and propulsion applications is with harmful effects of ingested dust, sand, volcanic ash, and other species entrained in turbine intake air. These species can adhere to TBCs and damage them through the formation of various comparatively low-melting point phases collectively referred to as “CMAS” due to their typical inclusion of such oxide components as calcia, magnesia, alumina, and silica. CMAS material generally melts around 1200° C. (about 2250° F.), which is below the surface temperature expected for TBC's in high-performance turbine components; once molten, the liquid CMAS infiltrates the cracks, pores, columnar grain boundaries, and open defects of TBCs and solidifies to form a glass when the TBCs cool to room temperature. As a result, the TBCs lose compliance and spall prematurely.
The industry standard 8YSZ material (zirconia stabilized with approximately 8 weight percent yttria) used for TBCs is particularly susceptible to degradation via CMAS. One technique to combat spallation resulting from CMAS ingestion involves TBC compositions with higher rare earth contents as compared to conventional TBCs. These high-rare-earth TBCs are designed to react with ingested CMAS and thereby limit its penetration. These high rare earth TBCs, however, have lower fracture toughness than conventional YSZ-based thermal barrier coatings, and thus, while attractive for some turbine applications, simply changing the chemistry of the coating may not be an ideal solution for all turbine designs.
As a result of the above, a need persists in the industry for thermal barrier coatings and related methods for fabricating coated components, where the coatings are resistant to CMAS ingestion (i.e., spallation resistant), include high strain tolerance, are scalable (i.e., compatible with large components), and are relatively inexpensive as compared with conventional thermal barrier coatings.